Build/Test Tools: Expose error message upon Gutenberg plugin activation failure.

[Bernhard Reiter]

In [54913], an end-to-end test was added to ensure that installing and activating the Gutenberg plugin didn't cause any fatal errors. (For the rationale behind this, refer to #57197.) Through its integration with GitHub's CI, this allowed catching code that would lead to fatals (e.g. duplicated function names) before it was committed.

The downside of doing this via an e2e test is that the error message shown in CI is fairly generic and thus somewhat easy to miss and/or ignore, even though WordPress does report the specific error that would cause an error when attempting to activate Gutenberg.

Thus, it seems preferable to implement the test differently to expose WordPress' error message. One way to go about this would be to add a separate GH workflow (yml file) and to use WP CLI to install and activate GB.

Description TBD

Trac ticket: https://core.trac.wordpress.org/ticket/59728

FYI @desrosj

Adding myself as a reviewer so that it's in my list to come back to!